Certainly Macbeth and Banquo's conversation about the "three weird sisters" serves multiple dramatic purposes:
1. Their conversation brings up the prophecy concerning both Macbeth and Banquo's possible futures. Their brief discussion reminds the audience and keeps the imagery of the witches fresh in their mind.
2. Banquo and Macbeth's conversation further reveals Macbeth's ability to lie and deceive. He tells Banquo that he has hardly thought of the witches' words, although the audience knows that is hardly the case! Macbeth comes across in this scene as looking so genial and the very picture of a perfect host and friend, which of course, serves as a bold contrast to the end of the scene in which he contemplates murdering the king.